This invention relates to monostable multivibrator circuits, and more particularly to a monostable multivibrator circuit for use in an FM detector circuit of a pulse count system.
A monostable multivibrator circuit usually has a time constant circuit which includes a capacitor, an amplifier circuit which receives an output from the time constant circuit and which has an input threshold voltage, a positive feedback circuit which is connected between an output end of the amplifier circuit and the time constant circuit, and a trigger terminal which is disposed in a circuit loop constructed of the time constant circuit, the amplifier circuit and the positive feedback circuit.
The output pulse width of this circuit is determined by a period of time during which the output signal level of the time constant circuit exceeds the input threshold voltage.
The inventors constructed a monostable multivibrator by employing differential amplifier transistors for the amplifier circuit and the positive feedback circuit. As a result, it has become possible to determine the input threshold voltage by a potential which is applied to the base of one of a pair of transistors of the differential amplifier. In this case, the input threshold voltage is decided by the external potential, not by the characteristic of an element itself as in the case of, for example, utilizing the base-emitter voltage of a transistor. Therefore, the input threshold voltage can be freely set and can be made a stable value without being influenced by a variation in the characteristic of the element itself.
Moreover, it has become possible to operate the differential amplifier transistor in the non-saturated state in such a way that the quantity of a voltage drop to appear in a collector load is limited by the current of a current source connected to the emitter of the transistor. Therefore, the lowering of a current gain and the accumulation of carriers in a collector region or a base region which are attributed to the saturated operation of the transistor do not occur, and an output signal of comparatively good waveform and quick response can be obtained.
Further, experiments and studies, however, have revealed that, in the monostable multivibrator employing the differential amplifier transistors as described above, the operation resetting time is susceptible to influences of noise, so that the output pulse width is not fixed.